Clark (U.S. Pat. No. 3,986,997, issued Oct. 19, 1976; and U.S. Pat. No. 4,027,073, issued May 31, 1977) is the basis for the first commercially useful silane/silica sol copolymer hardcoats for optical plastics. Clark is still serving as the basis for the commercially successful line of nontintable clear hardcoats for optical plastics offered by SDC Coatings of Anaheim, Calif. SDC is a joint-venture company of Swedlow (now re-named Pilkington Aerospace) and Dow Corning; the latter is the assignee of the Clark patents. SDC's commercial dipcoating formulation Silvue 101 is believed to be a higher % solids version of a Clark formulation (suited for dip coating); all Clark coatings are acidic pH.
Next came the GE Silicone entries into this field, as embodied in Frye (U.S. Pat. No. 4,299,746, issued Nov. 10, 1981; U.S. Pat. No. 4,324,839, issued Apr. 13, 1982; U.S. Pat. No. 4,413,088, issued Nov. 1, 1983) and Vaughn (U.S. Pat. No. 4,309,319, issued Jan. 5, 1982; U.S. Pat. No. 4,324,712, issued Apr. 13, 1982; U.S. Pat. No. 4,414,349, issued Nov. 8, 1983). All the abovementioned GE Silicone patents employ an alkaline pH aqueous colloidal silica dispersion, and their resulting coatings are alkaline pH, in contrast to Clark's. GE's commercial dipcoating formulation SHC-1200 is a commercially useful reference point.
Both the Clark/SDC and the Frye/Vaughn/GE coatings form heat-cured clear coating films of excellent scratch resistance (commonly tested with steel wool pads under load with a rubbing action, to simulate cleaning-type damage) and thereby offer essentially equivalent protection to the clear plastic substrates, when applied and cured at equal thicknesses. Both are sold at very high prices (on 100% solids basis, typically $80-110 per pound or more). Because of a greater propensity for autopolymerization inherent in the alkaline aqueous dipcoating solutions of the Frye/Vaughn/GE formulas, wherein polysiloxane bond formation via condensation reaction is favored over maximizing silanol stability (particularly as storage temperature or bath temperature is increased towards room temperature), use of acidic aqueous silica/silica sol copolymer, as in the Clark/SDC compositions, is preferred and therefore is the chemical basis for the present invention.
For users purchasing these liquid coatings, in addition to paying high prices for the coating material, there is the special handling required of refrigerated shipping and storage. Most particularly when dipcoating tanks are of large capacity, it is necessary to keep the contents chilled to minimize the rate of auto polymerization. "For maximum solution stability, Silvue abrasion-resistant coatings should be refrigerated at -18 to +4.degree. C. degrees (0 to 40.degree. F. degrees)", from SDC 5 Product Information Sheet #130-0, March 1993).
Another indirect cost to the user is the need to apply these water-based silane/silica sol copolymers--including dip, flow or spray coating operations and coating solvent-drydown areas--at very low humidity, typically a maximum of 35-40% R.H. Reference is made to GE Silicones product literature titled "SHC 1200 Optical Grade Abrasion-Resistant Silicone Hard Coat & SHP Primer" #CDS 4532 (May 1990), stating . . . "relative humidity controlled to 35% or less", and the previously-referenced Silvue literature , stating . . . "relative humidity of less than 40% is recommended. Variations from these conditions may result in blushing of the coating or a poor film formation". Thus, for optically and cosmetically satisfactory hardcoatings of spectacle lenses and other optically useful products, not only are the usual clean room conditions with HEPA-filtered, laminar airflow desirable, but the mandated maximum relative humidities are below the range which can be readily attained by ordinary HVAC interior air conditioning systems during year-round use. Supplemental dehumidification is needed, calling for expensive regenerative-desiccant-bed air-handling equipment which can easily exceed $100,000 capital, just to handle the volume of clean air recirculating within a fully-enclosed room or machine used for applying such coatings.
"Blush" can be defined as a hazy or foggy appearance within the otherwise-transparent cured film of the coating, which appears during drydown of the coating. Early versions of low-solids, high-water-content Clark coatings when applied, dried, and cured in ordinary, ambient conditions had very poor "blush" resistance; 30 grains of moisture or less was then specified as necessary in the air of the coating drydown area. Subsequently, the added step of azeotropic stripping of those 20% nominal solids was found to remove both excess alcohol and part of the excess water, so that the resulting stripped 35% solids Clark solutions had greater "blush" resistance & humidity tolerance. However, this stripping operation adds substantially to manufacturing costs.
Since the time of the Clark and Frye/Vaughn inventions, solvent-based colloidal silica sols have become commercially available. Potentially, they represent an alternative way of reducing the sensitivity of these silane/silica sol copolymer hardcoats to ambient levels of humidity, for increased "blush" resistance. Excess water in the liquid coating composition is defined as that which exceeds 100% stoichiometric amounts needed for hydrolysis of the alkoxy groups on the silane (3 moles of water per 1 mole of trialkoxysilane), in order to form the siloxane bond between the silane and the silica sol. Use of a solvent-based silica sol may actually require adding water to meet stoichiometric requirements for proper formation of the copolymer. However, the economic impact of replacing the aqueous silica sols with solvent-based silica sols is extremely negative, by at least a five-fold to ten-fold price factor. For example, Clark's preferred aqueous silica, Nalco 1034A (from Nalco Chemical; Naperville, Ill.), costs about $3 per pound on 100% solids basis, in drum quantities. By comparison, an alcohol-based sol of equivalent particle size from Nissan Chemical (New York City) costs over $75 per pound on equivalent 100% solids basis. A more recent one commercialized by Nalco still costs about $17 per pound on 100% solids basis, so for lowest costs, solvent-based silicas need to be minimized or eliminated in the coating formulation.
Notwithstanding this cost factor, solvent-based silicas have been copolymerized with various organofunctional (most commonly, epoxide attached by propyl to silicon) trialkoxysilanes, to make tintable dipcoatings for protecting plastic ophthalmic prescription spectacle lenses. Such coated lenses can be readily dip-dyed for desirable colorations. In these formulations, the organofunctional group attached to the silane is chosen for dye receptivity. Such coatings can be crosslinked by heat (examples in which Applicant was co-inventor are U.S. Pat. Nos. 5,013,608 and 5,102,695) or by ultraviolet as radiation (examples in which Applicant was co-inventor are U.S. Pat. Nos. 5,221,560 and 5,296,295). These specialized tintable Rx lens coatings are used on lower volume, high priced lenses, so they can tolerate higher-priced silicas and silanes than the general-purpose Clark hardcoatings.
Athough the Clark/SDC and the Frye/Vaughn/GE coatings provide "excellent" scratch resistance compared to prior art coatings, further improvements are desired and needed, not only for clear plastic substrates but also to protect the surfaces of non-transparent plastics, metals, woods, and even painted or finished articles. The deficiencies of the SDC/GE coatings are readily seen when they are rubbed with coarser steel wools than the finest, #0000, typically used in demonstrating their scratch resistance.
A convenient and effective embodiment of such a test is the Progressive Steel Wools (PSW) test. Successive grades of steel wools are rubbed 10 strokes (5 double rubs, back and forth) under very firm (at least 10 pounds) thumb pressure on the article to be tested, cross-wise of the alignment of the wool fibers. Depending on the hardness of the coating, and of the substrate it is on, there may be no visible scratches using the finest steel wool(s). However, coarser wools will produce marks--fine, medium, or coarse scratches. A coating is considered to have survived testing with a given grade of steel wool if there are a half-dozen or fewer scratches visible to the naked eye after such rubbing.
Another method of testing the hardness of coatings, unfortunately only applicable to flat substrates, is the Taber abrasion test. Using CS-10 wheels, Clark/SDC and Frye/Vaughn/GE coatings typically increase in haze about 5-7% after 500 revolutions of the grit-filled, abrasive wheels, which scuff the surface tested in a rolling/skidding motion. Harder coatings that are user-friendly are sought after.
A third method of testing the hardness of coatings, which is particularly used in the ophthalmic (prescription eyeglass lens) industry, is the AO tumble test. In this test, coated or uncoated articles, usually lenses, are tumbled with abrasive pads, grit, sawdust, etc. to simulate in-use wear. The test (details available from American Optical Lens Corp, Southbridge Mass.) was correlated at the time of its development with 1- and 2-year eyeglass lens wear tests using several materials and coatings. After tumbling and cleaning, the samples are compared against standards, and ranked, with a score of 10 being the highest.